MEXICO CITY -- The long quest to bring the Salvadoran military killers of six Jesuit priests to justice has received a significant boost, human rights activists say, with the sentencing of a former commander -- on unrelated charges in a faraway Boston court.

The activists say the 21-month jail term for former Col. Inocente Orlando Montano will give a Spanish court time to extradite him to Spain, where he would stand trial for the 1989 slayings.

“The important thing is to have him in custody,” said attorney Almudena Bernabeu of the San Francisco-based Center for Justice and Accountability, which has been championing the case.

Montano was sentenced Tuesday by a federal court in Boston after he entered a guilty plea on charges he lied on immigration forms about whether he had been given military training.

And while the conviction is related exclusively to immigration fraud, Montano’s past as a military commander implicated in the slayings and a host of other atrocities figured prominently in prosecution arguments during his sentencing hearing.

Montano is one of 20 military leaders during El Salvador’s civil war in the 1980s who was indicted in 2011 by a Spanish court on charges related to the killings of the priests. The Spanish court asserted the doctrine of universal justice, which allows a court in one country to prosecute egregious atrocities committed in another.

The priests, five of whom were Spanish nationals, were killed along with their housekeeper and her young daughter. They were rousted from their beds by soldiers during a guerrilla offensive in the Salvadoran capital.

A national truth commission, as well as a United Nations investigation, later determined that the killings were ordered by El Salvador’s military leadership -- and covered up by civilian officials -- who accused the priests of sympathies with the rebels.

Two officers were eventually convicted in a Salvadoran court but then freed in a 1993 amnesty.

Montano, now 70, will be the only person implicated in the killings serving jail time. Once the vice minister of public security in wartime El Salvador, he had been living a relatively quiet life in the Boston area for about 10 years before his arrest in 2011. He has denied any role in killing the Jesuits and the two others.

Over the years, human rights groups have attempted to find ways to reopen the Jesuit case, leading to a 2008 lawsuit filed in Spain by the Center for Justice and Accountability and a Spanish rights group, and then the 2011 indictment. However, Salvadoran authorities ignored the Spanish court’s demand that the named defendants be arrested.

The Center for Justice and Accountability, reacting to this week’s sentencing, urged U.S. authorities to act swiftly on Spain’s extradition request so that Montano’s trial for the killings can begin soon.

“The sentence of 21 months sends a message to human rights abusers that they cannot seek safe haven in the United States and avoid accountability for their actions,” the center said in a statement. “This is the moment of justice that the victims of the massacre deserve."

Bernabeu, the attorney, reached in Quito, Ecuador, where she was working on an unrelated case, also praised the Boston court’s ruling as “really, really good news,” adding: “We will get there, to justice, little by little.”